Thermally controlled solar reflector facet with heat recovery

ABSTRACT

A high concentration central receiver system and method provides improved reflectors and a unique heat removal system. The central receiver has a plurality of interconnected reflectors coupled to a tower structure at a predetermined height above ground for reflecting solar radiation. A plurality of concentrators are disposed between the reflectors and the ground such that the concentrators receive reflective solar radiation from the reflectors. The central receiver system further includes a heat removal system for removing heat from the reflectors and an area immediately adjacent the concentrators. Each reflector includes a mirror, a facet, and an adhesive compound. The adhesive compound is disposed between the mirror and the facet such that the mirror is fixed to the facet under a compressive stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/879,363 filed on Jun. 12, 2001. The disclosure of the aboveapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to solar power plants. Moreparticularly, the invention relates to a high concentration centralreceiver system having improved reflectors and a unique heat removalsystem.

[0003] As concerns over the environment, the deterioration of fuelsources, and energy efficiency continue to increase, solar power plantshave become the subject of worldwide attention. In the development ofsolar power plants, high concentration central receiver systems havedemonstrated a relatively high level of usefulness and are thereforequite popular. The conventional solar central receiver system has a“tower top” configuration in which a field of heliostats reflectsunlight onto a receiver mounted on a tower structure. The concentratedsolar energy on the receiver heats a fluid, such as oil or molten salt,to high temperatures. This energy is then transferred to a boiler/heatexchanger to produce steam, which then powers a steam turbine to produceelectricity. While this type of configuration has been shown to beuseful for power plants, other configurations have proven to be moreeffective for large power plants, especially when operated with highefficiency, combined cycle gas turbines powered by both natural gas andsolar energy.

[0004] One such configuration is the “tower reflector” configuration.One of the major features of this type of configuration is that specialparabolic concentrators are located on the ground beneath the tower andreflectors are coupled to the tower structure at a predetermined heightabove ground for reflecting solar radiation. The tower-mountedreflectors redirect sunlight from the heliostats, to the parabolicconcentrators which are located on the ground. The tower-mountedreflector is composed of a number of mirrors, coupled to a metallicfacet (or heat exchanger) for support. Each parabolic concentratortypically has a special quartz receiver into which the concentratedlight is directed. Air flowing through this receiver is air heated to ahigh temperature and then passes into the turbine combustion chamber,where it is further heated, before passing through the turbine toproduce electric power by turning the generator.

[0005] It is critical that the tower mounted reflectors provide thelight into the aperture opening of the parabolic concentrators at theappropriate angles under a wide variety of conditions. These conditionsinclude temperature changes, wind variations, solar insolation levels,sun angles, etc. It is very desirable that the concentrators have verylittle loss due to “spillage” under these conditions, becauseconventional systems make no use of this wasted heat. The towerreflectors must therefore achieve high optical quality at a low cost.The tower reflectors must also be able to withstand high concentrationsof solar energy and meet the optical requirements under a wide varietyof environmental conditions. It is therefore desirable to providereflectors having a good structural integrity and that are safe tooperate. It is also desirable to enable the reflectors to be adjustableand configurable such that there is minimal loss of reflected light fromthe heliostats in harsh environments and over several decades.

[0006] A particularly difficult aspect of conventional solar reflectorsrelates to high operating temperatures, cost and breakage. Specifically,while various facet designs and heat removal systems have been designedfor tower reflectors, a number of difficulties remain. For example, theconventional design has a small reflector area and uses small, hightensile strength, thick glass mirrors. Generally, these mirrors havebeen shown to be too costly for practical use in high temperaturecommercial applications. The conventional design is also prone tobreakage, since the glass is held by “clips” such that there are slightstresses built up in the glass under nominal conditions. It is thereforeeasy to understand that such systems can impose relatively high levelsof stress at local points under more severe conditions. For example,high stresses occur (especially when exposed to sand, dust and ice,since these can cause “ratcheting”) when the glass expands and contractsdue to exposure to diurnal cycles of high concentration irradiance, withhigh temperatures, followed by little or no irradiance and relativelycool temperatures.

[0007] The resultant expansion and contraction, with metal joints usedto hold the glass securely for good alignment, can result in high localstresses and breakage. Since the glass is not otherwise constrained, itcan fall, causing a significant hazard to equipment and personnel below.In particular, the falling glass can damage the high optical quality,relatively high cost Compound Parabolic Concentrators (CPCs) on theground. These thermal and stress related problems are exacerbatedfurther by the exposed clips, which can be subjected to over 50 to 100suns (i.e., 50 to 100 kW/m²). Since the metal has a relatively highsolar absorptivity, the operating temperature of the metal clips can bequite high, thus adding to the local thermal stresses already placed onthe facets by the direct, concentrated solar flux.

[0008] The conventional approach also does not provide for adequatethermal control to prevent ice buildup. Ice buildup on high structuresis a serious problem, since it can greatly increase the structural load,distort and damage the glass mirrors. If ice forms and falls onto theCPCs, further damage is likely to be caused to the system. It istherefore desirable to provide a design that ensures thermal control toprevent buildup.

[0009] Another aspect of the conventional design is that it uses arectilinear support structure. Such supports do not offer the torsionalstiffness inherent in geometries such as triangular shapes. The mass ofmaterial required, and the complexity of assembly (as well as cost) aretherefore higher than for other geometric shapes. For example, thetriangular design disclosed herein, is formed with a novel “geodesicdome” concept, that uses essentially equilateral struts arranged withnovel attachment fittings to allow easy assembly of the supportstructure and adjustment of the facets.

[0010] In the more general case, for certain applications, mirrors areheated by incident solar irradiance and/or heat flux. This heating cancause damage to the mirrors or to the support backing structure.Furthermore, the optical quality can be degraded by changes in theradius of curvature, increases in the surface slope error, damage to thereflective surface, or warpage. The problem is typically solved byeither selecting high tensile strength glass (at high cost), flowing astream of air over the mirror, or using a fluid coolant. It is importantto note that while conventional coolant-based heat removal systems aremoderately effective in sinking heat away from the reflectors, othershortcomings remain. For example, the “spillage” area immediatelyadjacent the concentrators is also a considerable source of heat.Removing heat from this area would both improve the operation of theconcentrators as well as provide additional heat to other systems (e.g.,residential/commercial systems). As already mentioned, extreme cold orice buildup can also cause problems. These problems include warpage ofthe facet or its support structure, changes in the facet cant angle,build-up of extremely high loads on the structure, or cracks in theglass. To mitigate concerns of extreme cold and ice buildup, anembodiment of the invention utilizing a fluid coolant heat recoverysystem can maintain adequate coolant temperatures to prevent formationof ice and protect the area from extreme cold.

[0011] In general, the mirrors must be adjusted to produce the beampositioning required by the application. This problem is typicallysolved by attaching multiple (most often, three) adjustable attachmentfittings to the back of the mirror assembly. Also, for certainapplications, the mirror assembly must be very light weight, and in someapplications the mirror must be mounted in a location where access isdifficult. For example, in the tower reflector case, the reflectors aremounted high above the ground (hundreds of feet high). Therefore, thereflectors must demonstrate exceptional long life and integrity, whileat the same time being light weight and inexpensive.

SUMMARY OF THE INVENTION

[0012] The above and other objectives are provided by a method and highconcentration central receiver system in accordance with the presentinvention. The central receiver system has a plurality of interconnectedreflectors coupled to a tower structure at a predetermined height aboveground for reflecting solar radiation. A plurality of parabolicconcentrators are disposed between the reflectors and the ground suchthat the concentrators receive reflected solar radiation from thereflectors. The central receiver system further includes a heat removalsystem for removing heat from the tower-mounted reflectors and an areaimmediately adjacent the parabolic concentrators. Removing the heat fromthe area immediately adjacent the concentrators improves operation ofthe concentrators and provides an additional source of energy that iseffectively wasted in conventional systems.

[0013] Further in accordance with the present invention, a reflector fora high concentration central receiver system is provided. The reflectorincludes a mirror, a facet, and an adhesive compound. The facet haswalls defining a coolant channel, where the cooling channel receives aheat conductive fluid. The adhesive compound is disposed between themirror and the facet such that the mirror is fixed to the facet undercompressive stresses. In the preferred embodiment, the glass mirror hasa compression stress value such that no part of the glass experiencestensile stresses. Generating compressive stresses in the mirror improvesthe strength and resistance to breakage because glass has a low tensilestrength. Pre-loading the glass in compression thus avoids the mostcommon failure mode for glass.

[0014] In another aspect of the invention, a method for fabricating areflector for a high concentration central receiver system is provided.The method includes the step of maintaining a mirror at a mirror bondingtemperature. A metal facet is maintained at a facet bonding temperature,where the facet bonding temperature is greater than the mirror bondingtemperature. The method further includes the step of bonding the mirrorto the facet with an adhesive, where an operating temperature for thereflector is less than the facet bonding temperature. Due to theinherent properties of glass and metal, the mirror is under compressivestresses at the operating temperature.

[0015] It is to be understood that both the foregoing generaldescription and the following detailed description are merely exemplaryof the invention, and are intended to provide an overview or frameworkfor understanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitutepart of this specification. The drawings illustrate various features andembodiments of the invention, and together with the description serve toexplain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The various advantages of the present invention will becomeapparent to one of ordinary skill in the art by reading the followingspecification and sub-joined claims and by referencing the followingdrawings, in which:

[0017]FIG. 1 is a diagram of a tower structure with a high concentrationcentral receiver system according to the present invention;

[0018]FIG. 2 is a diagram of a heat removal system in accordance withthe principles of the present invention;

[0019]FIG. 3A is a top view of a reflector according to one embodimentof the present invention;

[0020]FIG. 3B is a side view of the reflector shown in FIG. 3A;

[0021]FIG. 3C is a cross sectional view taken along lines 3 c-3 c inFIG. 3A;

[0022]FIG. 4 is a top view of a reflector having cooling fins accordingto an alternative embodiment of the present invention;

[0023]FIG. 4B is a side view of the reflector shown in FIG. 4A;

[0024]FIG. 5A is an end view of a reflector having cooling finsaccording to an alternative embodiment of the present invention;

[0025]FIG. 5B is a cross sectional view taken along lines 5B-5B shown inFIG. 5A;

[0026]FIG. 6A is a cutaway top view of a reflector showing threeembodiments of a turbulence generating system according to the presentinvention;

[0027]FIG. 6B is a cross sectional view taken along lines 6B-6B shown inFIG. 6A;

[0028]FIG. 6C is a cross sectional view taken along lines 6C-6C shown inFIG. 6A;

[0029]FIG. 6D is a cross sectional view taken along lines 6D-6D shown inFIG. 6A;

[0030]FIG. 7 is a top view of a reflector having facet walls that definea plurality of channels according to an alternative embodiment of thepresent invention;

[0031]FIG. 8A is a top view of a plurality of interconnected reflectorswherein each reflector has facet walls that define a plurality ofcoolant channels according to an alternative embodiment of the presentinvention;

[0032]FIG. 8B is a diagram showing coolant flow paths for theconfiguration shown in FIG. 8A;

[0033]FIG. 9 is a side view of a plurality of reflectors, where thereflectors have honeycomb shaped stiffening plates;

[0034]FIG. 10 is a side view showing attachment fittings according toone embodiment of the present invention; and

[0035]FIG. 11 is a side view showing attachment fittings according to analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numeralswill be used throughout the drawings to refer to the same or like parts.

[0037] Turning now to FIG. 1, a high concentration central receiversystem 20 is shown in greater detail. The receiver system 20 has aplurality of interconnected reflectors 22 (or reflector assemblies)coupled to a tower structure 24 at a predetermined height above groundfor reflecting solar radiation 26. A plurality of concentrators 28 aredisposed between the reflectors 22 and the ground such that theconcentrators 28 receive reflected solar radiation from the reflectors22. A heat removal system 30 (or spillage collector) removes heat fromthe reflectors 22 and an area immediately adjacent the concentrators 28.

[0038] Thus, FIG. 1 shows the basic beam down optics central receiverconcept with the reflectors 22 located atop a high tower structure 24(of the order of several hundred feet). The solar irradiance isconcentrated on the reflectors 22 from a field of heliostats 29. For a10 Megawatt (thermal) system, approximately 1300 heliostats, eachapproximately 9 to 10 square meters in area, are needed. The solar flux,or irradiance, incident on the tower can easily range up toapproximately 50 kW/m², or higher, which could cause high temperatures(several hundred degrees F. in the glass and higher in any exposedsupport structure). For this reason, the mirrors of the reflectors 22are cooled to prevent changes in optical properties, warpage, breakage,or separation of the glass and supporting structure. The heat removalsystem 30 also prevents other forms of damage and degradation, such asoverheating of the support structure, loss of silver, acceleratedcorrosion at high temperature, deposition of foreign materials (withsubsequent hot spots caused by the incident highly concentrated solarflux), etc. The mirrors are also overlapped, to minimize concentratedsunlight from overheating the support structure behind the mirrors.

[0039]FIG. 2 shows the heat removal system 116 block diagram. A heattransfer fluid, such as water, mixed with an anti-freeze (e.g.,propylene glycol), is contained in a tank, 120. A pump 122 pumps theliquid through a pipe to the tower reflector structure where it passesthrough each of the mirror assemblies 118. The temperature is raisedfrom initial inlet temperatures in the tank of the order of 10 to 30° C.to outlet temperatures of the order of 50 to 90° C. The heat transferfluid then flows through a pipe down to the “spillage collector” 124that surrounds the compound parabolic concentrators 126. The fluidtemperature is further increased, to temperatures of the order of100-120° C. The heat transfer fluid then passes through an optional heatexchanger capable of transferring additional waste heat and furtherraising the temperature. The heat transfer fluid then passes through aheat exchanger suitable for the selected end-use of the collected wasteheat. For example, the waste heat can be used to heat and/or desalinatewater for residential or industrial use. It can also be used in anorganic Rankine cycle (ORC) turbine generator, similar to geothermalheat recovery, to produce electricity. The heat transfer fluid exits theheat exchanger at a temperature on the order of 40 to 60° C. and can befurther cooled by passing through coils submerged in a cooling pond orin a cooling tower prior to entering the holding tank 120.

[0040] Although a series flow path is shown, other flow configurationscan be used. For example, the flow out of the pump can be split suchthat part flows directly to the spillage collector 124 and part flows tothe tower reflector 118.

[0041] It should be noted that there are substantial pressuredifferences due to the hydrostatic head. The pressure through the mirrorassemblies is relatively low, compared to the pressure at ground level.This provides two advantages. First, the lower pressure in the mirrorassemblies minimizes distortion of the mirrors. However, the distortionthat occurs essentially causes the mirror to form a slightly convexmirror surface, as seen from below. This convex mirror surface is closerto the ideal hyperbolic shape, and this improves the mirror opticalquality.

[0042] Second, the higher pressure at ground level allows the heattransfer fluid to remain a liquid or a two-phase mixture at moderatelyhigh temperatures, thus improving heat transfer. As long as thetwo-phase flow remains in the bubbly and slug flow regimes, the annularflow regime, or the initial region of annular to mist transition, theheat transfer coefficient will be high. However, as the fluid becomes amist flow or forced convection vapor flow, the heat transfer coefficientdrops substantially. The lower heat transfer coefficient increases therequired temperature difference in heat exchanger size and cost and isthus to be avoided. The higher pressure helps keep the heat transferfluid in the high heat transfer coefficient region.

[0043] Turning now to FIGS. 3A and 3B, it can be seen that in oneembodiment each reflector 34 includes a mirror 36 and a facet 38 havingwalls 40 defining a coolant channel 41. The coolant channel 41 isconnected to one or more adjacent reflectors such that the adjacentreflectors receive a heat conductive fluid from the heat removal systemand pass the fluid through the channel. As shown in FIG. 3C, an adhesivecompound 37 is disposed between the mirror 36 and the facet 38 such thatthe mirror 36 is fixed to the facet 38 under a compressive stress. FIGS.3A and 3B show two views of a reflector 34 having a single pass coolantfluid path. Although the preferred design has a triangular geometry, thepresent invention is not constrained to this shape. Other shapes (e.g.,rectangular, hexagonal, circular, or square) may therefore be usedwithout parting from the spirit and scope of the invention.

[0044] As already noted, the mirror 36 is bonded to the facet 38 (orbacking plate). Note that the mirror 36 may be glass or glass laminatedto a protective substrate (sheet steel or plastic) and may be one pieceor several pieces. Mirrors with the silver reflector coating on the backside of the glass and with a protective substrate are commonly referredto in the industry as second surface mirrors. This assembly is held in afixture during the adhesive curing cycle to obtain the required finishedcontour (e.g., flat, cylindrical, spherical, parabolic, or hyperbolic).The portion of the drawing in FIG. 3B shows the flow path that is formedby the facet design. The facet 38 is embossed, stamped, or hydroformedmetal or reinforced plastic (match die molded, or spray/hand lay-up) toyield the cavities for the cooling media to pass through. Note thatreference is made to “cooling” where, as will be explained below,certain applications require a heated fluid.

[0045] It is also important to note that FIGS. 3A and 3B show simple “L”shaped fluid fittings 42, which may be welded, brazed, or bonded, etc.to the facet 38, depending on the application and materials selected.For support, three attachment fittings 44 are secured to the facet 38.While the illustrated attachment fittings 44 are simply threaded studsthat may be welded, brazed or bonded to the facet 38, other options willbe detailed later.

[0046] It will be appreciated that the flow path may be designed inseveral different ways to achieve the desired effect. For example, FIGS.3A and 3B show a simple, single pass approach. The depth and width ofthe cavity should be optimized to supply the necessary cooling byproviding the required fluid velocity and pressure drop for the system(especially when a large number of mirror facets 38 are connected inseries). Modifications to this simple configuration will be describedbelow.

[0047]FIG. 4A and FIG. 4B show the same simple, single pass facet 38 asshown in FIGS. 3A and 3B, with cooling fins 46 attached. This designoffers the additional advantage of ensuring a uniform temperature of themirror 36, due to the coolant flow, while rejecting the heat to theatmosphere. This approach could be used when it is not necessary torecover the waste heat. The cooling fins 46 of this design wouldpreferentially be made from metal in order to have good heat transfer,but plastic materials (especially with good thermal conductivity) arealso permissible. For facets 38 that are formed from metal, welding orbrazing of the cooling fins 46 would provide efficient heat transfer.For applications that are weight dependent, the fins undesirably addweight to the assembly, but they offer very good stiffness advantages.Therefore the facet 38 may be made thinner while maintaining equivalentfacet stiffness.

[0048]FIG. 5A and FIG. 5B illustrate detailed views of the preferredcooling fin design. For facets made of steel, stainless steel, aluminum,copper, etc., spot welding or roll-spot welding of the cooling fins 46are simple and efficient methods for providing good heat transfer acrossthe junctions 48. Welding also provides adequate strength to maintainthe stiffening qualities of the assembly. Bonding with a thermallyconnecting adhesive is also a viable option.

[0049] To provide the coolant, the mirrors 36 are connected to a fluidcooling loop via fittings on a back surface 36 a. Each mirror facet 38has a coolant path provided by an embossed or ribbed structure whichprovides the flow channels required. The configuration of the flow canbe single or double pass, but other options are possible. Thissupporting structure for the coolant flow may be formed sheet metal(steel, stainless steel, aluminum, etc.) or a plastic material (sheetmolding compound, spray or hand layup fiberglass, graphite/epoxy, etc.).

[0050] It should be noted, however, that the best combination of glassand facet from the standpoint of reducing the tensile stress in theglass is the selection of materials that have similar coefficients ofthermal expansion (CTE) to minimize tensile stress. Alternatively, forthe situation in which compressive loads are imposed on the glass 36,the facet 38 should preferentially have a CTE slightly greater than thatof the glass. Typically, fiber glass or steel comes closest to matchingthe coefficient of thermal expansion of the various types of glass thatare candidates for this design, and both of these have CTEs slightlyhigher than glass. Aluminum has CTE significantly higher than glass, butcare must be taken to ensure that the stresses induced do not causeexcessive deformation. Other reflective surfaces are potentialcandidates, but due to the lack of long life and low cost materials, thehigh reflectivity of silvered glass is preferred. Thus, although thepreferred embodiment uses either steel, aluminum, or fiberglass for thefacet 38, the present invention is not restricted to these materials.

[0051] Another aspect of the design that reduces the stress on the glass36 is the use of an adhesive that has the correct combination ofcompliance and thermal conductivity. In fact, various adhesives satisfythese criteria, as specified further below. The glass 36 can also bebonded to a thin, protective sheet metal or composite material having agood thermal conductivity characteristic, using a double backed sheetadhesive. Alternatively, the adhesive can be applied directly to theglass back surface and/or to the facet by various well known techniques(spray, curtain coat, roller coat, brush, etc.). The glass mirror 36without the protective laminate layer is then bonded (with a thermallyconductive adhesive) to the triangular facet 38 that provides thecoolant loop for thermal control.

[0052] Regardless of the material selected for the facet 38, theinternal coolant loop flow path is designed to be large enough in crosssectional area, short enough in path length, and the flow rate selectedto be low enough such that flow resistance is not excessive. At the sametime the facet 38 facilitates maintaining conditions for effective heattransfer to control the temperature of the glass, and the resultantstresses, to acceptable levels. In particular, the glass 36 temperatureis uniform across its surface to minimize stress concentrations, and ismaintained at temperatures of approximately 50 to 100 degreesCentigrade, or lower. Various means may be employed to enhance the heattransfer.

[0053] FIGS. 6A-6D illustrate three different methods of addingturbulent generating features to the design to further enhance heattransfer. Heat transfer is greatly improved when the coolant is in astate of turbulent flow. For this reason, some means of disturbing thelaminar flow conditions may be required for a specific application,especially in the vicinity of the thermally conducting fins. The upperexample of FIG. 6B shows how a molded facet 50 could have inner fins 52formed on the interior to force the fluid into a turbulent state. Notethat this increases the pressure drop (and therefore the need for astronger panel design). The fins 52 may be contoured such that the base52 a of each fin 52 stiffens the facet 50, but the tip 52 b is flexibleenough to “flutter” and cause additional turbulence, especially in thevicinity of the inner surface of the heated mirror 54.

[0054] Where a mirror 56 is bonded to a metal substrate 58, as shown inthe lower left example of the FIG. 6C (thence bonded to a facet 60, thesubstrate 58 may be formed with inner projections 62 to alternate withsimilar deformations 64 in the facet 60. The lower right hand portion ofFIG. 6D shows how a facet 66 may be formed to have a serpentine path forthe fluid as it passes around projections 68 from formed dividers 70 ofthe facet 66.

[0055]FIG. 7 shows a reflector 72 having a two-path flow for the fluid.This approach has two particularly important advantages. First, thisflow configuration tends to better average the facet temperature.Secondly, it allows the inlet and exit fluid fittings 74 to be inline.Although this is of little advantage for one facet, the facetinterconnect becomes much simpler for a group of facets as seen in FIG.8A to follow.

[0056]FIG. 8A and FIG. 8B illustrate how multiple reflectors 72 of thetwo-path flow type may be connected to achieve efficient cooling of aportion of a large facet array 76. With this configuration, a simpleplastic or rubber tube may be used as a fluid fitting 74 for low tomoderate pressure systems. This resilience allows for adjustment of theindividual facets (via various attachment fittings) of the array 76without straining the reflectors. The insert sketch of FIG. 8B shows asimplified flow pattern 78 for this type of interconnect.

[0057]FIG. 9 shows a facet design with a honeycomb-shaped stiffeningplate 80 supporting a mirror 82. The plate 80 increases the stiffnesssuch that higher pressure drops across facets 84 can be accommodatedwithout causing the mirror 82 to deform in a short radius of curvatureconvex shape (looking at the mirror) that could tend to spread thereflected beam by an excessive amount.

[0058]FIG. 10 shows a variation on the simple attachment design shown inFIGS. 3 and 9 for which the attachment fitting 44 is a simple, threadedstud that forms a coolant feed fitting 90. Here an adjusting mechanism86 is also used for the fluid inlet/exit paths. A facet 88 has thecoolant feed fitting 90 welded, brazed, or bonded to one of a pluralityof raised flow paths 92. The flow paths 92 should be positioned so thatthey are parallel to each other to minimize alignment problems at thetime of the installation. A pair of lock nuts 94 and two sets ofspherical washers 96 are used to attach the facet 88 to a supportstructure 98. Preferably, the structure 98 has an oversized hole toallow for alignment variations. The coolant feed fitting 90 shown is astraight threaded tube, but other designs may be used for specificapplications. For example, a ball stud can be used to accommodate largeangular variations. Since a triangular facet will typically requirethree points of support (to minimize distortion of the mirror), one ofthe coolant feed fittings 90 could be a “dummy” and not be drilledthrough, or formed from a tube, for a flow passage as the other two are.However, there are other flow configurations that can benefit fromhaving one or two inlets with a corresponding two or one outlets. Thisis especially true at the outer periphery of the set of facets, wherethe flow enters an outer annular ring.

[0059]FIG. 11 illustrates a variation to the above embodiments. Here afitting 100 is welded, brazed, or bonded to a facet 102. The design issimilar to an AN fluid fitting except that the normal 37 degree flaredend has been modified to have a spherical end. A swivel feed line 106 ismachined to have a matching spherical (concave) end. A sealing washereffects a leak tight joint while allowing for a minimal amount ofvariation of alignment. A matching spherical surface is machined on theback side of the flange of the swivel feed line 106. The standard “BNut” secures the joint when tightened but allows for movement whenloosened. Lock nuts 110 secure the reflector 112 to a support structure114. Note that spherical washers are not required for this design. Also,the tubing sleeve that is normally used with AN fittings, is not neededfor this design.

[0060] It should further be noted that this design provides severaleffects that mitigate the glass tensile stress, when properly used.First, the adhesive between the glass 36 and the protective backing(optional) can be sufficiently compliant such that the difference incoefficient of thermal expansion (CTE) does not induce as high a tensilestress as for the case of intimate contact. Second, the adhesive used tobond the glass-protective sheet laminate to the triangular facet 38 (orheat exchanger) adds a further degree of compliance that reduces thestress imposed on the glass. Third, the adhesive(s) and the protectivesheet have a temperature drop such that the facet 38 is at a lowertemperature than the mirror 36, for incident solar energy, which tendsto reduce stresses, as will be explained further below. Fourth, theadhesive and/or protective sheet thermal conductivity can be selected toprovide the preferential temperature gradient properties. Stress in theglass can also be minimized by proper selection of the adhesives andoptional protective sheet, for a variety of glass and triangular facetthermal coefficient of expansion characteristics.

[0061] Another innovation of the present invention that further improvesthe optical performance, structural integrity, life and overall costeffectiveness, lies in the method of forming the completed facet so asto build compressive stresses into the mirror, especially for mirrorshaving glass that is exposed to high concentrations of solar energy orhigh loads (i.e., wind). Since the facets may be exposed to high solarirradiance, internal pressure, and temperature, all of which can inducetensile stress in the glass, facet bonding is performed at an elevatedtemperature of the steel, while controlling the temperature of the glassto be less than that of the steel. The CTE of steel typically exceedsthat of the type of glass applicable to mirrors of interest. Therefore,the steel is heated while cooling the vacuum table/mandrel which is incontact with the glass. The shape of the vacuum table/mandrel can beflat, concave or convex, depending on the required shape of the mirror.Heat applied to the steel accelerates the rate of curing of the adhesivein contact with the steel, which decreases the fabrication time and thusreduces cost. The high temperature bonding and subsequent cooling to theoperational temperature range (typically, −20 degrees F. to 120 degreesF.) ensures that there is little or no tensile stress induced in theglass. The same process can be used with aluminum and other metals, asdesired.

[0062] Minimizing the temperature difference between the glass and thetriangular support structure also helps to avoid stress buildup. This isaccomplished by using an adhesive, with the proper thickness, that has ahigh conductivity, usually obtained by loading it with aluminum or othermaterials. The proper selection of adhesive, glass, and supportstructure material can be accomplished such that the glass tensilestress (and the delaminating stress on the adhesive bond) are wellwithin the requirements. For example, glass typically can be stressed intension practically indefinitely at levels of the order of 500 psi orless. The normal breaking tensile stress for glass is approximately 2000to 3000 psi, or higher, depending on the type of glass. It is thereforeimportant to minimize tensile stress, or preferably, eliminate it byplacing the glass in compression as disclosed above.

[0063] It is also important that the glass conduct the heat to thefacet/heat exchanger, such that the waste heat in the thermal fluid canbe recovered and used in power generation, process heat, space heating,etc. Accordingly, the appropriate combination of materials having therequired thicknesses, compliance, thermal conductivity, and strength areused to meet these combinations of appropriate compliance, thickness,and thermal conductivity. Since steel has a thermal coefficient ofexpansion (CTE) close to, but higher than that of glass, and since theglass is at a temperature slightly higher than the steel, there is atendency for the glass and steel to expand at approximately the samerate. Specifically, assume that Tf is the temperature at which the glassand steel are bonded together (both glass and steel are at the sametemperature during this process), Tog is the operating temperature ofthe glass, Tos is the operating temperature of the steel, and CTEg andCTEs are the coefficients of thermal expansion of the glass and steelrespectively. Then, when the glass and steel are operated at atemperature different from the temperature at which they were bondedtogether, there will be some stresses set up in the glass and steel.This stress difference is proportional to the difference in the productof CTEs and temperature difference (Tos-Tf) and the product CTEg andtemperature difference (Tog-Tf). At sufficiently high tensile stressvalues, the glass would break. But, since CTEg is less than CTEs, andTog-Tf is typically greater than Tos-Tf, there is a tendency for thetensile stress to be reduced or eliminated.

[0064] The method of forming the facet with the built in compressivestress in the glass can be accomplished several ways according to thepresent invention. Specifically, under one approach the steel ismaintained at a higher temperature than the glass during bonding. Forexample, a heated fluid, heat lamps, electrical heaters, or other suchmeans can be used to heat the steel facet/heat exchanger. Conversely,the glass is cooled by the form on which it is placed. This form, ormandrel, maintains a certain curvature or flatness (e.g., a vacuum tableor other surface having a cooling fluid, refrigerating coils). The steelcan be at a temperature of, say, 100 degrees Centigrade—well above itsoperational temperature in the field—while the surface in contact withthe glass can be maintained at a temperature less than, or equal to,room temperature. Thus, when the steel/glass laminate is bonded togetherand then removed from the bonding table, the steel would tend tocontract far more than the thin glass (typically, of the order of 1 mm),thus putting the glass in compression.

[0065] In addition, if the glass were maintained at a temperature lessthan its operating temperature during the bonding process, then theglass would tend to expand, but being constrained by the steel, wouldencounter additional compressive stresses. This latter effect, however,though beneficial from a stress standpoint, could decrease the rate atwhich the adhesive is cured depending on the type of adhesive and itscure rate vs. temperature properties. On the other hand, there are manyadhesives for which this would not be a problem, and high cure ratescould be achieved even at temperatures well below room temperature.Since glass in compression is approximately as strong as steel, thecompressed glass would have greatly improved integrity and life.

[0066] It is important to note that the adhesive bond is preferablyfully cured, since otherwise the compliance and “flow” of the adhesivewould tend to reduce the compressive stress in the glass. It is alsopreferred to have the bonding process completed as quickly as practicalto have a high production rate from the tool to reduce costs. Therefore,for adhesives requiring elevated temperatures for rapid curing, theglass is not cooled to a low temperature, since this tends to lower therate at which the adhesive bond is cured, especially close to the glassinterface. Rather, the steel is heated to a temperature above themaximum operating temperature, but well below the acceptable operatingtemperature for the adhesive. Again, the preferable approach is to havethe adhesive formulated for rapid cure even at temperatures at, or wellbelow, room temperature. Rapid curing allows significant temperaturedifferences between the glass and steel, which in turn allows greatercompressive loads to be imposed on the glass.

[0067] By minimizing the tensile stresses, or preferably, imposingcompressive stresses in the glass, the fluid can be used at a highertemperature and at a higher pressure to cool the mirrors. The higherfluid temperature and higher pressure tends to cause the glass to bowout, forming a convex shape. By being able to operate the mirrors at ahigher temperature, higher efficiency is achieved in the powerconversion system (e.g., Organic Rankine Cycle turbine) or greaterbenefits are achieved for process heat or space heating applications. Bybeing able to operate at a higher pressure, higher flow rates can alsobe achieved. Thus the height of the channels is diminished, less coolantis used, and the weight of the facet with the coolant is reduced—all ofwhich reduces costs.

[0068] It should also be noted that the shape of the mirror facet ispreferably triangular. This is a key feature of the preferredembodiment. The triangular shape allows the design to be used with a“geodesic dome” supporting structure. A novel modification has been madeto the usual approach for joining the struts that form the geodesicshape needed to have the right optical shape (usually, a hyperboloid).This design has a plate which allows the facets to be affixed to theplate by adjustment screws or fixtures. The plate is sized such that thefacets can be overlapped, to maximize the reflected energy to thereceiver below the tower mounted reflector, and to minimize exposure ofthe tower structure behind the facets to intense radiation from theheliostats below.

[0069] Returning now to FIG. 1, the use of the facet in a representativesystem application is highlighted as follows. In this case,approximately 450 square meters of reflectors 22 are arranged to form ahyperboloidal shape, located on top of the tower structure 24. Thereflectors redirect energy delivered from a field of heliostats 32(approximately 1300 heliostats of approximately 10 square meters area inthis example) back to ground level. This redirected energy will deliverapproximately 10 Megawatts of thermal energy (for the specific fielddesign selected), which is then transformed into approximately 3 to 5Megawatts of electrical power by the through conversion in a gas turbineor combined cycle system, using for example, a steam turbine and OrganicRankine bottoming cycle.

[0070] There are various uses of the waste heat collected at the towerreflector, which is at a temperature of the order of 50 to 100 degreesCentigrade. These uses include but are not limited to industrial processheating, space heating, or conversion to additional electric power in alow temperature Organic Rankine cycle turbine, as is commonly done withgeothermal plants and in certain co-generation applications.

[0071] In particular, conversion of waste heat to additional electricpower provides unique economic benefits. For example, the peak flux onthe facets is typically of the order of 50 kW/m² or higher, and theaverage flux is of the order of 5 to 10 kW/m². A pump, located on theground, delivers a water/ethylene glycol (or equivalent) solution to thetop of the tower structure 24. The heat transfer fluid then splits intoa combination of series and parallel flows through the reflector facetsin such a fashion as to maintain the facet operating temperature atapproximately 80 to 100 degrees C. while keeping the pressure drop to apreferred level.

[0072] The outlet of the reflectors 22 is returned to ground level tointerface with the Organic Rankine bottoming cycle via a heat exchanger.The additional power generated in the bottoming cycle from use of thepumped loop cooled facets generates sufficient cash flow to pay for theactive thermal management system and the facets. In this case, we haveassumed a relatively low price for the electricity sales of$0.10/kilowatt hour. Therefore, in markets where solar electrical poweris priced at values of the order of $0.10/kilowatt hour or above (i.e.,so-called Green Power Pricing markets, or in subsidized or PortfolioStandards markets) we can expect that the thermally cooled facet designdisclosed herein will more than pay for the entire tower reflectorconcept through waste heat recovery.

[0073] Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

What is claimed is:
 1. A method of forming a high concentration centralreceiver system for a solar power plant comprising the steps of: forminga dome structure with a plurality of interconnected reflectors supportedby a tower structure at a predetermined height above a ground surfacefor reflecting solar radiation; disposing a plurality of concentratorsbetween the plurality of reflectors and the ground surface for receivingreflected solar radiation from the reflectors; and employing a heatremoval system operative to provide a heat conductive liquid to thereflectors for removing heat from the plurality of interconnectedreflectors.
 2. The method of claim 1, further comprising the step ofemploying a field of heliostats disposed around a periphery of the towerstructure for reflecting solar radiation to the plurality ofinterconnected reflectors.
 3. The method of claim 1, wherein the step offorming a dome structure comprises the steps of: forming a triangulargeometry with each of the plurality of interconnected reflectors; andoverlapping the plurality of interconnected reflectors such that avarying overlap is defined between the plurality of interconnectedreflectors.
 4. The method of claim 1, wherein using a reflectorcomprises using a reflector formed from a mirror secured to a facet, thefacet further being formed with a plurality of walls to form a coolantflow channel.
 5. The method of claim 4, further comprising the steps of:defining a coolant channel between the plurality of walls; andconnecting the coolant channel to one or more adjacent reflectors suchthat the adjacent reflectors receive the heat conductive liquid from theheat removal system and pass the liquid through the coolant channel. 6.The method of claim 5, further comprising the step of disposing anadhesive compound between the mirror and the facet such that the mirroris fixed to the facet under a compressive stress.
 7. The method of claim5, further comprising forming a plurality of cooling fins on the facetfor removing heat from the conductive liquid.
 8. The method of claim 5,further comprising using the facet to generate a turbulence fordisturbing laminar flow of the heat conductive liquid.
 9. The method ofclaim 8, wherein the facet includes a metal backing and the mirrorincludes a metal substrate, the metal backing being coupled to the metalsubstrate of the mirror, the metal backing and the substrate havingalternating projections extending into the coolant channel.
 10. Themethod of claim 8, further comprising forming the facet to include aplurality of fins extending into the coolant flow channel.
 11. Themethod of claim 4, further comprising forming the facet with a pluralityof alternating projections extending into the coolant flow channel. 12.The method of claim 5, further comprising the step of disposing astiffening plate between the mirror and the facet of each of theplurality of reflectors.
 13. The method of claim 12, wherein disposingthe stiffening plate comprises disposing a stiffening plate having ahoneycomb shape.
 14. The method of claim 5, further comprising using aplurality of fluid flow attachment fittings with the facet for couplingthe reflector in fluid communication with the structure.
 15. The methodof claim 5, further comprising using a plurality of fluid flow coolantfittings to place the coolant channel in fluid flow communication withcoolant channels of the adjacent reflectors.
 16. A method for forming ahigh concentration central receiver system for a solar power plantcomprising: forming a dome-like structure from a plurality ofreflectors; supporting the dome-like structure above a ground surfacewith a tower; using a plurality of concentrators disposed elevationallyin between the dome-like structure and the ground surface to receivereflected solar radiation from the reflectors; and using a heat removalsystem in fluid communication with the tower and the reflectors forcirculating a coolant through the tower and the reflectors to cool thetower and the reflectors.
 17. The method of claim 16, further comprisingemploying a field of heliostats on the ground surface, and disposed soas to generally encircle the tower, for concentrating solar radiation tothe plurality of reflectors.
 18. The method of claim 16, wherein formingthe dome-like structure comprises forming the dome-like structure from aplurality of triangular shaped reflectors disposed adjacent one another.19. The method of claim 16, wherein forming the dome-like structurecomprises forming the dome-like structure from a plurality of reflectorseach having a shape comprising at least one of: a rectangle, a hexagon,a circle and a square.
 20. The method of claim 16, wherein forming thedome-like structure comprises using a plurality of reflectors, eachhaving a mirror and a facet, with the facet having a plurality of wallsforming a coolant flow channel.
 21. The method of claim 20, furthercomprising: connecting the coolant channel to one or more adjacentreflectors such that adjacent ones of the reflectors receive the heatconductive liquid from the heat removal system and pass the liquidthrough the channel; and disposing an adhesive compound between themirror and the facet such that the mirror is fixed to the facet under acompressive stress.
 22. The method of claim 21, wherein the mirror has acompressive stress imposed by the facet during manufacture of thereflector.
 23. A method of forming a high concentration solar receiver,comprising: forming a dome-like structure from a plurality ofindependent reflectors; supporting the dome-like structure above aground surface; interconnecting the reflectors for fluid flowcommunication; using a concentrator to receive reflected solar radiationfrom the reflectors; and using a heat removal system to circulate acooling fluid flow through the reflectors to cool the reflectors duringuse.
 24. The method of claim 23, further comprising overlapping thereflectors when forming the dome-like structure.
 25. The method of claim23, further comprising using a tower to support the dome-like structure.26. The method of claim 24, further comprising forming the tower with afluid flow path and circulating the cooling fluid through the tower tothe reflectors.
 27. The method of claim 23, further comprising using aplurality of heliostats supported on the ground surface, and disposedabout the tower, to reflect solar radiation from the sun toward thereflectors.
 28. A method of forming a high concentration solar receiver,comprising: forming a dome-like structure from a plurality ofreflectors; forming each of the reflectors to include a serpentine fluidflow path; supporting the dome-like structure above a ground surface;using a plurality of heliostats strategically disposed about a peripheryof the dome-like structure to reflect solar radiation from the suntoward the reflectors; collecting solar radiation reflected from thereflectors at a point elevationally below the dome-like structure toconcentrate the solar radiation; and cooling the reflectors with a fluidflowed through the serpentine flow path of each of the reflectors. 29.The method of claim 28, wherein supporting the dome-like structure abovea ground surface comprises supporting the dome-like structure with atower.
 30. The method of claim 29, further comprising using the fluid tocool the tower.
 31. The method of claim 29, further comprising formingeach of the reflectors in a shape comprising at least one of: atriangle, an octagon, a rectangle, a circle and a square.